Liberty Union High School District / Overview



Name: _____________ANSWER KEY________________ Date: ______________________________ Period: _______Radioactivity and Half-Lives Review WorksheetWhat is a half-life?The time it takes for half of the material you started with to decay.If we start with 400 atoms of a radioactive substance, how many would remain after one half-life? 200 After 2 half-lives? 100 After 3 half-lives? 50 After 4 half-lives? 25If we start with 16 grams of a substance, how much will remain after 3 half-lives?16 8 4 22 grams after 3 half-lives.A sample of carbon-14 has been decaying for 22,920 years and is now 35.0 grams. What was the size of the original sample? (The half-life of carbon-14 is 5,730 years)22920/5730 = 4 half livesYou will need to work backwards35 70 140 280 560. gramsHow old is a skeleton sample if the current amount of carbon in the bones is 3.125%? (Assume you started with 100%)100% 50 25 12.5 6.25 3.1255 half lives x 5,730 = 28,650 yearsIf you started with 120 grams of a radioactive substance, and now you have 15 grams, how many half-lives have passed?120 60 30 153 half-livesThe half-life of isotope X is 2.0 years. How many years would it take for a 4.0 mg sample of isotope X to decay and have only 0.50 mg of it remain?4.0 2.0 1.0 0.503 half lives x 2 years = 6.0 yearsThe half-life of Po-218 is three minutes. How much of a 200 atom sample will remain after 15 minutes?15 minutes/3 = 5 half lives200 100 50 25 12.5 6.25 atomsThe half-life of Au-198 is 2.69 days. How long does it take a 180g sample to decay to 1/8 its original mass?1 ? ? 1/8 3 half lives x 2.69 days = 8.07 daysWhat is the half-life of a radioactive sample if 100.0 grams of it decays to 12.5 grams in 24.3 hours?100 50 25 12.524.3/3 half lives = 8.10 hoursHow long will it take a sample of 131I to decay to 12.5% assuming its half-life is 8.07 days?100% 50 25 12.53 half lives x 8.07 = 24.2 daysSodium-25 was to be used in an experiment, but it took 3.0 minutes to get the sodium from the reactor to the laboratory. If 5.0 mg of sodium-25 was removed from the reactor, how many mg of sodium-25 were placed in the reaction vessel 3.0 minutes later if the half-life of sodium-25 is 60 seconds?3 minutes / 60 seconds = 3 half lives52.5 1.25 0.625mgActinium-226 have a half-life of 29 hours. If 10.0 g of actinium-226 disintegrates over a period of 145 hours, how many mg of actinium-226 will remain? **You need to do dimensional analysis to solve this one! **145 / 29 = 5 half lives10.0g 5.0 2.5 1.25 0.625 0.3125g4765866-650875000.3125 g x 103 mg = 312.5 mg1 g ................
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